Hello,
While attempting to add an extra index to an existing config, I was
able to (accidentally!) delete the attribute, but not add it back:
dn: olcDatabase={1}hdb,cn=config
changetype: modify
delete: olcDbIndex
modifying entry "olcDatabase={1}hdb,cn=config"
dn: olcDatabase={1}hdb,cn=config
changetype: modify
add: olcDbIndex
olcDbIndex: objectClass eq
-
ldapmodify: wrong attributeType at line 8, entry
"olcDatabase={1}hdb,cn=config"
Is this a bug, or have I done something stupid?

There seems to be something wrong with your LDIF. Probably a trailing
whitespace somewhere (likely on the "add: olcDbIndex" line)?